Tool Briefing: Charge Light Check-out

The trouble code in this customer’s vehicle, P1638, indicates incorrect charging voltage. Any time you examine a charging system that’s controlled by the Powertrain Control Module (PCM), the very first tool you need is a service information system. Different battery technologies require different charging voltage, and the charging system may be programmed to work only with the factory-installed battery type. For conventional flooded lead/acid batteries, the charging system would be programmed to produce a nominal 14.5 volts. With other batteries, such as Absorbed Glass Mat (AGM), Valve Regulated Lead/Acid (VRLA), Gel Cells and other types of sealed batteries, different charging voltages may be required. Before doing anything, check your vehicle service information system for battery specifications and charging system specifications.

The next step is to check the condition of the battery. Checking voltage is a good place to start, but a battery load test is just as important. The carbon pile load tester, which has been used for over a century, is a circuit that converts electrical energy into heat. We can vary the resistance in the circuit to vary the current draw or ‘load’ placed on the battery. Newer testers can also test an alternator’s voltage output, current output and the diodes.

Load testers use an inductive pick-up to read battery and alternator current: when a coil of wire is wrapped around a straight wire, current passing through the straight wire will induce a voltage in the coil. That voltage ‘picked up’ is directly proportional to the current in the straight wire, so by measuring the induced voltage, the tester can calculate the current through that circuit.


With a little practice and imagination, a standard inductive pick-up current probe can be used for testing low current circuits too. Wrap a jumper wire ten times around your hand to make ten loops of wire. Secure the loops together, add the jumper wire to the circuit and clamp the current probe through the loop. The probe will now see ten times the current running through the circuit.

VAT Testing

The easiest way to test a charging system is under load with a Volt/Amp Tester (VAT). There are many on the market, and several are small enough to carry in the vehicle while it’s being driven. Some will generate a printout of diagnostic results, and the same data can also be downloaded to a computer.

On some of the newest vehicles, the PCM will command almost no charge at idle or when there is almost no electrical load. A VAT tester connects to the battery terminals and draws current, forcing the alternator to increase current output to keep the battery charged. Not all VAT testers are capable of putting a full load on the alternator, but when combined with an oscilloscope ripple test, even 50 percent load is enough to tell the story.

Duty Cycle Test

If the alternator is capable of producing current, measuring the field duty cycle will tell you if the alternator is being commanded to produce current. That command comes from the PCM, which controls the ‘on time’ of the alternator field current, and you can see it with a Digital Volt/Ohm Meter (DVOM) that reads duty cycle. When field current is turned on continuously, that’s a 100 percent duty cycle and the alternator will produce its maximum rated current output. The alternator will produce half its rated output when field current is turned on for only 50 percent of one full cycle.


What’s a ‘full cycle?’ When a DVOM is first connected and set to read duty cycle (Hertz or Hz%), it reads the minimum and maximum voltage on that circuit. It sets a trigger point half way between the min and max readings. One full excursion above that trigger point, then below it, and back up to it again, is one full cycle. The percentage of each cycle that the voltage is above that mid point is defined as ‘on time.’ On some meters, you can manually set the trigger point.

There are two different places to check alternator duty cycle; within the PCM and at the alternator field circuit connections. To see the duty cycle commanded by the PCM, you need a scan tool. Not all scan tools can access that data, but if yours does it probably shows up on the Live Data (Mode 1) screen.


To check the field circuit duty cycle manually, connect the DVOM’s ground lead to chassis ground and backprobe the positive lead to the alternator’s field circuit connecter. Set the meter to read duty cycle and check the reading at different engine speeds. If there’s no change, turn on some lights. If there’s still no change, backprobe the field circuit at the PCM connector. If you get a reading now, there’s probably something wrong with the wiring. You can also try testing this circuit with a low current probe.

Low Current Probe

When using an ammeter or DVOM to read current, the meter must be connected into the circuit. A low current probe allows you to measure current using a DVOM without disconnecting any wires. It’s the perfect tool for measuring battery current draw when the engine is not running, otherwise known as a battery drain test.

The probe uses a Hall Effect sensor, a flat, rectangular semiconductor chip with two circuits and a wire leading to each side; north, south, east and west. The north/south circuit carries an extremely low but constant DC current, and the east/west circuit measures voltage. Normally, current flows straight through the length of the chip, and no voltage is detected in the measuring circuit. When a magnetic field comes close to the chip, it “bends” the current flow towards the east or west side of the chip, and the measuring circuit detects voltage. The stronger the magnetic field, the greater the affect on current flow through the chip, and the higher the voltage in the measuring circuit. The measured voltage is extremely low level, but it’s accurate enough to measure the strength of that magnetic field.


When DC current flows through a wire, a magnetic field is generated around the wire. The low current probe we’re using to detect battery drain actually detects the magnetic field around the battery cable, and that current reading is reported. But to get an accurate reading, the probe must be used correctly. A low-current Hall Effect sensor is extremely sensitive to current flow direction and ambient electrical energy fields.

There are many opinions about what is considered normal key-off battery current draw. Most techs agree that when all of the vehicle’s control units are in ‘sleep mode’ (up to 30 minutes after turning the key off), 50 milliamps is OK, 75 milliamps is too high. However on the newest models those numbers may vary, and some car manufacturers actually publish a specification for key-off battery current draw.

Now that the PCM is in control of the charging system, there’s more information available about its performance. Since the alternator has already been replaced on this vehicle, damaged wiring or corroded connections are prime suspects. To find these, the only tools you need are time and your eyes, but only after you’ve confirmed the battery really is not charging.



Search our Online Product Guide for more tools: